1. Field of the Invention
The present invention relates to an improved heat treatment apparatus and more specifically to an apparatus for producing a C/C composite, i.e., a carbon/carbon composite material, graphite, etc. used as an electrode base material for fuel cells, etc. by applying heat treatment to the workpieces to be treated under a non-oxidizing atmosphere of more than 2000.degree. C., preferably more than 2500.degree. C.
2. Description of the Prior Art
According to conventional methods, the electrode base materials of fuel cells are produced, for example, by solidifying carbon fibers into a sheet-like shape, using a binding resin, such as a phenolic resin, and then calcining the workpiece in a highly purified non-oxidizing atmosphere at a temperature between about 1000.degree.-3000.degree. C.
Because of the purpose for which this kind of electrode is used, the electrode requires characteristics, such as oxidization resistivity, chemical resistivity, electrical corrosion resistivity as well as conductivity and gas permeability, etc., and these characteristics are heightened as the above calcining temperature is increased. For example, the desirable calcining temperature for electrode materials for phosphoric acid type fuel cells is said to be above 2200.degree. C., preferably more than 2500.degree. C.
Conventional industrial furnaces for such high temperature heat treatment call for methods, such as induction heating, Tammann heating, Acheson heating, resistance heating, etc.
In the induction heating and Tammann heating methods, graphite materials are often used as the heating element under a non-oxidizing atmosphere of 2000.degree.-2500.degree. C. or above, as shown in JP-B-No. 59-7803, JP-B-No. 59-25936, etc. The heating elements used for these heating methods are generally tube-like in shape, with the inside spaces of these tubes serving as an effective heating area. For this reason, due to the limitation in the size of the graphite to be used as the heating element, the size of workpieces to be calcined is limited. For instance, it is difficult, using conventional methods, to calcine large square electrode materials of more than 1 m.times.1 m in width and length for fuel cells for practical use.
On the other hand, the Acheson heating method, which is a heating method generally used for calcining graphite materials, calls for heating graphite powder by directly applying current to it, thus applying heat treatment to workpieces buried in the graphite powder. However, as the result of an experiment in which several hundreds of electrode materials of 1 m in width, 1 m in length and 0.1 mm in thickness each were stacked with graphite material loaded on them, and placed in a graphite box, and in which the calcining treatment according to the Acheson heat treatment method was applied to them, it has been found that the yield is quite bad as the materials stick to each other and wrinkles form on the surface of the materials. Possible reasons for such results include separation of impurities from graphite powder and the adhesion of these impurities to the base materials and uneven generation of heat to the base materials, etc. Further, this method has another disadvantage in that due to the large thermal capacity of graphite powder, it takes quite a long time, for instance 1-3 weeks, for heating or cooling.
According to the resistance heating method, a highly purified atmosphere can be obtained if a heat resistant material which contains no impurities nor reacts with the atmospheric gas is used as the insulating material, and the uniformity and gradient of the temperature can be set at any desirable condition, by arranging the heating elements at suitable locations facing the workpieces, because the heating elements are completely separate from the workpieces to be heated, and the workpieces are heated, not by means of generation of heat in themselves, but by radiating heat from the heating elements completely separate from the workpieces. Therefore, this method is widely used.
However, in cases where workpieces in the shape of a wide sheet or block are calcined by high temperature, especially more than 2500.degree. C. in a non-oxidizing atmosphere, the heat resistance method will present problems as described below, because the furnaces of the heat resistance method usually use graphite group heat insulating materials. Since the graphite group heat insulating materials do not have so much thermal capacity, due to their small bulk density, and they greatly absorb the heat radiated from the heater and greatly release the heat to the outside because their emissivity is about 1, it is necessary to set the heater temperature much higher than the treatment temperature for the workpieces. Therefore, in order to obtain high treatment temperature, especially of higher than 2500.degree. C., by the resistance heating method using a graphite heater, the temperature of the heater often has to be increased to the high vaporization rate temperature of the graphite. Vaporization of graphite reduces the life span of the heater; for example, when the treatment temperature is 2500.degree. C., the life span of the heater is reduced to between several tens and several hundreds of hours, several months at the longest.
In the case of treating workpieces where a binding resin is used, such as the electrode base materials of fuel cells, substances generated from thermal decomposition of the binding resin at the time of treatment must be taken out of the treatment chamber. If the resin is a phenolic resin, this substance consists of combustible matter, such as cresol, xylenol, hydrocarbon, carbon monoxide, etc., which, at room temperture, are in various modes ranging from the gaseous phase to the liquid phase. As for the heat insulating material, it is the general practice to use a graphite group insulating material, such as graphite felt, at least at the innermost side of a treatment chamber when the treatment temperature is 2500.degree. C. or higher. However, such practice presents some disadvantages including, shortened life span due to vaporization of heat insulating material and decrease of heat insulating capacity caused by the thermal decomposition of matter, which is generated from the workpieces to be treated and solidifies in the low-temperature part of and adheres to the graphite felt.
Further, for this kind of heat treatment, inert gases, such as N.sub.2, Ar, He, etc., are used as the non-oxidizing atmosphere. In case N.sub.2 is used, however, there is a problem that the heater and heat insulating material made of graphite material will deteriorate considerably, because N.sub.2 and graphite react together at high temperatures of more than about 2500.degree. C. And when Ar is used, it is generally known that electric gas discharge (dielectric breakdown) is generated at high temperatures.
Furthermore, in applying heat treatment in a non-oxidizing atmosphere to workpieces which will generate a large amount of inflammable matter, conventional apparatuses will present problems as described below:
(a) As there is no means of separating a non-oxidizing atmospheric area which contains combustible gas generated (hereinafter referred as combustible gas atmospheric area) from an oxidizing atmospheric area in the exhaust gas treatment department, there is a possibility that the pressure relativity between the two atmospheric areas may be reversed and the oxidizing gas will counterflow into the combustible gas atmospheric area due to various complications, such as operation conditions, operation error, malfunction or other accidents, thus causing generation of a mixed atmosphere exceeding the explosion limit or damage to the products by the oxidizing gas.
(b) As conventional apparatus are not structured to cope with changes in the amount of gas generated through thermal decomposition and sudden changes in gas volume due to heat expansion resulting from changes in the treatment temperature, it is difficult to keep the treatment pressure within a specified range. There is a fear that gas pressure may exceed the pressure resistance capacity of the treatment chamber, depending on treatment conditions.
(c) Conventional apparatus are so designed that when increasing or decreasing treatment temperature or when stopping the apparatus, exhaust gas always passes through the exhaust gas incineration equipment, even though the density of the gas generated through thermal decomposition has become lower than the environmental safety standard level. In order to prevent this low density thermal decomposition matter from condensing in the cool part of the passage in the exhaust gas system and from adhering to and accumulating in the system, it is necessary to continuously operate the exhaust gas incineration equipment in spite of the fact that the density of the gas generated through thermal decomposition is low enough, and thus energy efficiency is bad.